1. Field of the Invention
The present invention relates generally to an information recording medium for recording, erasing, rewriting or reproducing information optically or electrically and a method for manufacturing the same.
2. Background Information
One type of conventional information recording medium is a phase-change type information recording medium. Information is recorded, erased, and rewritten on the phase-change type information recording media by utilizing a phenomenon that a recording layer (a phase change material layer) changes between a crystalline phase and an amorphous phase for recording, erasing, rewriting or reproducing information. Among the phase-change type information recording media, a type that utilizes a laser beam for recording, erasing, rewriting or reproducing information optically is called an optical phase-change type information recording medium. This optical phase-change type information recording medium has a recording layer with a phase change material that changes between the crystalline phase and the amorphous phase by heat from a laser beam. A difference of reflectance between the crystalline phase and the amorphous phase is detected so as to read the information from the recording layer. Among the optical phase-change type information recording media, a rewritable optical phase-change type information recording medium is one in which recorded information can be erased or rewritten. In other words, a rewritable optical phase-change type information recording medium has a recording layer whose initial state is usually in the crystalline phase, and a recorded state that is usually in the amorphous phase. In general, when recording information, the recording layer is melted and cooled abruptly with irradiation of a laser beam having a higher power (a recording power) than that for erasing, so that the irradiated portion is changed to an amorphous phase. On the other hand, when erasing information, the recording layer is warmed and cooled gradually with irradiation of a laser beam having a lower power (an erasing power) than that for recording, so that the irradiated portion is changed to a crystalline phase. Therefore, it is possible to erase recorded information while recording or rewriting new information on the rewritable optical phase-change type information recording medium by irradiating a recording layer with a laser beam whose power is modulated between a high power level and a lower power level. In addition, among the optical phase-change type information recording media, a write once optical phase-change type information recording medium is one in which information can be recorded only once and the recorded information cannot be erased or rewritten. The write once optical phase-change type information recording medium has a recording layer whose initial state is usually in the amorphous phase. When recording information in this type of information recording medium, the recording layer is heated and then cooled gradually with irradiation of a laser beam having a high power (a recording power), so that so that the irradiated portion is changed to the crystalline phase.
Instead of applying a laser beam as described above, electric energy (for example, current) can be applied for another type of medium so as to generate Joule heat and to change a state of the phase change material of the recording layer for recording information. This type of medium is called an electric phase-change type information recording medium. In this electric phase-change type information recording medium, a state of the phase change material of the recording layer is changed between the crystalline phase (low resistance) and the amorphous phase (high resistance) by Joule heat generated by applying current. A difference of resistance between the crystalline phase and the amorphous phase is detected so as to read the information from the recording layer.
One example of an optical phase-change type information recording medium is a 4.7 GB/DVD-RAM that is disclosed in Japanese Unexamined Patent Publication No. 2001-322357. The 4.7 GB/DVD-RAM basically has a seven-layered structure, as shown in FIG. 12, in which the seven-layered structure includes a first dielectric layer 2, a first interface layer 3, a recording layer 4, a second interface layer 5, a second dielectric layer 6, an light absorption adjusting layer 7 and a reflecting layer 8 on the substrate 1 in this order viewed from the laser incident side to form an information recording medium 12. The first dielectric layer 2 and the second dielectric layer 6 have an optical function of adjusting an optical distance and increasing light absorption efficiency of the recording layer 4 so as to increase a difference of reflectance between the crystalline phase and the amorphous phase so that the signal intensity is enlarged.
The first dielectric layer 2 and the second dielectric layer 6 also have a thermal function of insulating thermally the substrate 1, a dummy substrate 10 or the like that is heat-sensitive from the recording layer 4 that becomes high temperature when information is recorded. The material of (ZnS)80(SiO2)20 (mol %) that has been used is a superior dielectric material that is transparent and has a high refractivity, a low thermal conductivity for a good thermal insulation, good mechanical characteristics and good resistance to humidity. Note that film thickness values of the first dielectric layer 2 and the second dielectric layer 6 can be determined precisely by calculation in accordance with a matrix method so as to satisfy conditions that quantity of light reflected by the recording layer 4 changes substantially between the crystalline phase and that the amorphous phase and that light absorption by the recording layer 4 becomes large.
As a material of the recording layer 4, a fast crystallization material containing (Ge—Sn)Te—Sb2Te3 is used, which is obtained by replacing Ge with Sn partially in the GeTe—Sb2Te3 quasi-binary line of phase change materials. The GeTe—Sb2Te3 quasi-binary line is a mixture of compounds GeTe and Sb2Te3, that realize not only good initial recording and rewriting performance but also a good record conservation property (an indicator whether or not a recorded signal can be reproduced after a long conservation) as well as a good rewriting conservation property (an indicator whether or not a recorded signal can be erased or rewritten after a long conservation).
The first interface layer 3 and the second interface layer 5 have a function of preventing material transfer that may occur between the first dielectric layer 2 and the recording layer 4, as well as between the second dielectric layer 6 and the recording layer 4. This material transfer is a phenomenon in which S (sulfur) diffuses into the recording layer when the recording layer 4 is irradiated with the laser beam repeatedly when recording or rewriting information where the first dielectric layer 2 and the second dielectric layer 6 are made of (ZnS)80(SiO2)20 (mol %). When sulfur diffuses into the recording layer, the reflectance of the recording layer 4 is deteriorated, so that repeated rewriting performance is deteriorated. In order to prevent this deterioration of the repeated rewriting performance, it has been proposed in Japanese Unexamined Patent Publication No. 10-275360 to use a nitride containing Ge for the first interface layer 3 and the second interface layer 5.
The light absorption adjusting layer 7 has a function of adjusting a ratio Ac/Aa of a light absorption rate Ac when the recording layer 4 is in the crystalline state to a light absorption rate Aa when the recording layer 4 is in the amorphous state, so as to suppress a distortion of a rewritten mark shape.
The reflecting layer 8 has an optical function of increasing quantity of light that is absorbed by the recording layer 4 and a thermal function of dispersing heat rapidly that is generated in the recording layer 4 for rapid cooling so that the recording layer 4 becomes the amorphous state easily. In addition, the reflecting layer 8 also has a function of protecting a multi-layered film from surrounding environment.
Utilizing the techniques described above, superior rewriting performance and high reliability have been realized in the process of commercializing the 4.7 GB/DVD-RAM.
Note that a material that contains no sulfur can be used for the dielectric layer in order to prevent deterioration of the repeated rewriting performance. It has been proposed in Japanese Unexamined Patent Publication Nos. 2003-091870 and 2003-228881 that a material that mainly contains SnO2 can be used as one of the materials for the dielectric layer and that contains no sulfur.
Furthermore, in recent years, various techniques have been researched to provide larger storage capacity for information recording media. For example, a technique for achieving high density recording by using a blue-violet laser having a shorter wavelength than that of a conventionally used red laser or using a thin substrate on the side from which a laser beam is incident and a lens having a large numerical aperture NA to reduce the spot diameter of the laser beam has been researched. When the spot diameter is reduced for recording information, an area irradiated with the laser beam is restricted to a smaller area. As a result, a power density absorbed by the recording layer increases so that volume variation increases. Therefore, the material transfer phenomenon can occur more easily, and the repeated rewriting performance may be deteriorated if a material such as ZnS—SiO2 containing sulfur is used in contact with the recording layer.
In addition, another technique has been researched, in which an optical phase-change type information recording medium is provided with two information layers. The two information layers are recorded/reproduced with a laser beam incident from one of the two information layers. This technique, which uses two information layers, can achieve about twice the recording density. An information recording medium including two information layers on/from which information is recorded/reproduced from one side can hereinafter be referred to as a two-information recording-layer medium. Information is recorded and/or reproduced on the two information layer by using a laser beam that enters from one side. It has been proposed in Japanese Unexamined Patent Publication Nos. 2000-036130 and 2002-144736. In this two-information recording-layer medium, a laser beam that has passed one information layer that is closer to the laser beam incident side (hereinafter referred to as a first information layer) is used for recording and reproducing information on the other information layer that is farther from the laser beam incident side (hereinafter referred to as a second information layer). Therefore, a film thickness of the first information layer is made very thin so that the transmittance thereof is enhanced. However, when the recording layer becomes thin, an influence of material transfer from a layer that contacts with the recording layer increase. Therefore, if a material containing sulfur is used such as ZnS—SiO2 that is in contact with the recording layer, then the repeated rewriting performance may be deteriorated rapidly. In such a case, one method for preventing the deterioration of the repeated rewriting performance is to dispose interface layers made of a nitride containing Ge on both sides of the recording layer so as to reduce the influence of the material transfer in the same manner as the case of 4.7 GB/DVD-RAM.
However, in the case of optical phase-change type information recording medium for recording information in high density by reducing a spot diameter of the laser beam, larger energy (laser power) is applied to the recording layer when recording information. Therefore, if the conventional interface layer contains a nitride containing Ge, then heat generated in the recording layer may destroy the film of the interface layer. As a result, the diffusion of sulfur from the dielectric layer cannot be prevented, and the repeated rewriting performance may be deteriorated rapidly.
In addition, a nitride containing Ge has high thermal conductivity. Therefore, heat can disperse easily if the interface layer is thickened for suppressing the diffusion of sulfur from the dielectric layer. As a result, recording sensitivity may be deteriorated.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved information recording medium. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.